Laser-induced structuring of substrate surfaces

ABSTRACT

In one aspect, the present invention provides a method of processing a substrate, e.g., a semiconductor substrate, by irradiating a surface of the substrate (or at least a portion of the surface) with a first set of polarized short laser pulses while exposing the surface to a fluid to generate a plurality of structures on the surface, e.g., within a top layer of the surface. Subsequently, the structured surface can be irradiated with another set of polarized short laser pulses having a different polarization than that of the initial set while exposing the structured surface to a fluid, e.g., the same fluid initially utilized to form the structured surface or a different fluid. In many embodiments, the second set of polarized laser pulses cause the surface structures formed by the first set to break up into smaller-sized structures, e.g., nano-sized features such as nano-sized rods.

GOVERNMENT SUPPORT

This invention was made with government support by Army Research Officeunder grants W911NF-05-1-0341 and W911NF-06-1-0097 and by NationalScience Foundation under grant DMR-0213805. The U.S. government hascertain rights in this invention.

BACKGROUND

The present invention relates generally to methods of processingsubstrates by applying short laser pulses to the substrates and theresultant structured substrates.

A variety of techniques are known for generating small-sized features onsolid substrate surfaces, such as semiconductor surfaces. Some examplesof these techniques include lithography and chemical etching thatprovide control over the shape and the size of the structures. However,such techniques are often complex and costly. Pulsed laser-assistedetching is another technique for fabricating small structures directlyonto a substrate. The typical sizes of such structures are, however,larger than the laser wavelength.

Accordingly, there is a need for improved methods for generatingsmall-sized features on a substrate surface.

SUMMARY

The present invention relates generally to methods of structuringsubstrates and the resultant structured substrates in which a pluralityof micron-sized and/or submicron-sized features are formed in a topsurface layer of the substrate. In some cases, such structuring of asubstrate, e.g., a semiconductor substrate, can be achieved byirradiating a substrate surface, while in contact with a fluid (e.g., aliquid), with a plurality of polarized short radiation pulses, where thepolarization of some of the pulses is different than that of the others.For example, in some embodiments, the substrate surface can beirradiated with a first set of linearly polarized short laser pulseswhile exposed to a liquid to form a plurality of features in a top layerthereof. Subsequently, the structured surface can be irradiated with asecond set of linearly polarized short laser pulses with a differentpolarization axis (e.g., a polarization rotated by 90°) so as to breakup the surface features formed by the first set into smaller-sizedfeatures.

In one aspect, the present invention provides a method of processing asubstrate, e.g., a semiconductor substrate, by irradiating a surface ofthe substrate (or at least a portion of the surface) with a first set ofpolarized short laser pulses while exposing the surface to a fluid togenerate a plurality of structures on the surface, e.g., within a toplayer of the surface. Subsequently, the structured surface can beirradiated with another set of polarized short laser pulses having adifferent polarization than that of the initial set while exposing thestructured surface to a fluid, e.g., the same fluid initially utilizedto form the structured surface or a different fluid. In manyembodiments, the second set of polarized laser pulses cause the surfacestructures formed by the first set to break up into smaller-sizedstructures, e.g., nano-sized features such as nano-sized rods.

In some cases, a surface density of the nanosized structures (e.g.,nanosized rods) can be in a range of about 1×10⁸ cm⁻² to about 1×10¹¹cm⁻², e.g., in a range of about 5×10⁹ cm⁻² to about 5×10¹⁰ cm⁻².

In some cases, the surface structures generated after the application ofthe first set of pulses are in the form of ripples extending across thesurface with a spacing that is substantially equal to the wavelength ofthe incident radiation or less than that wavelength. In some cases inwhich the radiation pulses in the first set are linearly polarized, thelong axis of the ripples is substantially aligned with the direction ofthe polarization. In some cases, the second set of pulses can break upthe ripples into nano-sized rods, e.g., having a diameter in a range ofabout 50 nm to about 200 nm and a height in a range of about 50 nm toabout 500 nm.

In a related aspect, the short radiation pulses have pulse widths in arange of about 10 femtoseconds (fs) to about few hundred (e.g., 500)nanoseconds (ns), and preferably in a range of about 100 fs to about 1picosecond, and more preferably in a range of about 100 fs to about 500fs. In many embodiments, the pulse wavelengths (central wavelength of apulse) in either set can be, e.g., in a range of about 400 nanometers(nm) to about 800 nm. Further, the energy of a pulse in either set canbe in a range of about 10 microjoules (P) to about 400 microjoules,e.g., in a range of about 60 μJ to about 100 μJ.

In another aspect, the pulses are directed to the surface, e.g., focusedonto the surface such that the fluence of a laser pulse at the substratesurface in either set is less than about 40 kJ/m². By way of example,the pulse fluence can be in a range of about 0 kJ/m² to about 30 kJ/m²,and preferably in a range of about 1 kJ/m² to about 4 kJ/m². In someembodiments, the fluence of the laser pulses in the second set are lessthan the fluence of the laser pulses in the first set. For example, thefluence of the laser pulses in the second set at the substrate surfacecan be less than the respective fluence of the pulses in the first setby about 50%, or preferably by about 25%.

In another aspect, the fluid utilized in at least one of the irradiationsteps, and in many cases in both irradiation steps, comprises a liquid.Some examples of such a liquid can include, without limitation, water oralcohol. In some cases, the liquid can comprise an electron-donatingaqueous solution, e.g., an aqueous solution of sulfuric acid. In somecases, the fluid in contact with the substrate surface in at least oneof the irradiation steps comprises a gas. Some examples of a gassuitable for use in the practice of the invention include sulfurhexafluoride (SF₆), nitrogen (N₂), H₂S or air.

In a related aspect, in the above method, the radiation pulses in oneset are linearly polarized with a polarization axis along a givendirection while the radiation pulses in the other set are linearlypolarized with their polarization axis along a different direction. Forexample, in some cases, the polarization axes associated with theradiation pulses in two sets can be orthogonal to one another. In othercases, the radiation pulses are circularly polarized with thepolarization sense in one set being opposite to that in the other set(e.g., the polarization sense in one set can be clockwise and in theother set counter-clockwise).

The above method can be applied to different types of substrates. By wayof example, the substrate can be a semiconductor substrate, such as,silicon, germanium, doped or undoped. In other cases, the substrate canbe any of glass, metal, or insulator.

In another aspect, a method of structuring a substrate, e.g., asemiconductor substrate, is disclosed that includes applying a pluralityof short laser pulses, e.g., pulses having durations in a range of about20 fs to about 20 ns, to a surface of the substrate while the surface isin contact with a gas, e.g., SF₆ or N₂, to generate a plurality ofmicron-sized features, e.g., spikes, on the surface, e.g., in a toplayer of the surface.

Subsequently, the structured (roughened) surface can be subjected to twoseparate sets of short laser pulses having different polarization whilein contact with a liquid, to generate a plurality of nano-sized features(e.g., nanosized rods). By way of example, the roughened surface can beirradiated with one set of polarized radiation pulses having a temporalduration in a range of about 10 fs to about a few nanoseconds (e.g., 10ns) while the surface is in contact with a liquid, such as water, andsubsequently be irradiated with another set of polarized short radiationpulses with a different polarization while the surface is in contactwith a liquid.

In some cases, the nano-sized features can be superimposed on themicron-sized features. By way of example, the nano-sized features can bein the form of nano-sized rods (e.g., rods having diameters in a rangeof about 50 nm to about 200 nm and heights that are less than about 1000nm) that extend from the micron-sized features, e.g., in a upwardlynormal direction.

In another aspect, a method of structuring a semiconductor substrate isdisclosed that includes applying a plurality of polarized short laserpulses to the substrate surface such that at least two of the pulseshave different polarizations while the surface is exposed to a liquid soas to generate a plurality of nano-sized structures on the surface.

In another aspect, a semiconductor substrate is disclosed that includesa plurality of micron-sized structures disposed in a top surface layerof the substrate, and a plurality of nanosized structures that aresuperimposed on the micron-sized structures.

In another aspect, the invention provides a method of processing asubstrate by irradiating at least a portion of the substrate surfacewith circularly polarized short radiation pulses while exposing thesurface portion to a fluid, e.g., a liquid, so as to generate aplurality of structures, e.g., micron-sized and/or submicron-sizedstructures, in a top surface layer of the substrate.

In a related aspect, the circularly polarized pulses can have a temporalduration in a range of about 20 fs to about a few hundred nanoseconds,e.g., in a range of about 20 fs to about 500 fs. Further, the pulses canhave a fluence in range of about 2 kJ/m² to about 40 kJ/m².

Further understanding of the invention can be obtained by reference tofollowing detailed description in conjunction with the associateddrawings, which are described briefly below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart depicting various steps in an embodiment of amethod according to the teachings of the invention for structuring asubstrate surface,

FIG. 2A is an SEM image of a silicon substrate surface after itsexposure to a set of polarized femtosecond laser pulses while in contactwith water, where the pulses have a fluence of 25 kJ/m² at the substratesurface,

FIG. 2B is an SEM image of a silicon substrate surface after itsexposure to a set of polarized femtosecond laser pulses while in contactwith water, where the pulses have a fluence of 5 kJ/m² at the substratesurface,

FIG. 2C is an SEM image of a silicon substrate surface after itsexposure to a set of polarized femtosecond laser pulses while in contactwith water, where the pulses have a fluence of 3 kJ/m² at the substratesurface,

FIG. 3A is an SEM image of the structured silicon surface shown in FIG.2C after its exposure to another set of polarized femtosecond pulseshaving a different polarization while in contact with water, where thepulses exhibited a fluence of 0.5 kJ/m² at the substrate surface,

FIG. 3B is another SEM image of the structured surface shown in FIG. 3Aat a different resolution,

FIG. 3C is another SEM image of the structured surface shown in FIG. 3Aobtained at a vantage point different than that utilized to obtain FIG.3A,

FIG. 3D is a cross-sectional SEM image of the structured siliconsubstrate whose surface image is shown in FIG. 3A,

FIG. 4 schematically depicts an apparatus suitable for practicingmethods of processing substrates in accordance with the teachings of theinvention, and

FIGS. 5A-5C are SEM images of a silicon substrate surface which wasinitially roughened and was then subjected to a two-step irradiationprocess in accordance with an embodiment of a substrate processingmethod of the invention, at different magnifications.

DETAILED DESCRIPTION

The present invention relates generally to methods for structuringsubstrates, e.g., semiconductor substrates, in which a plurality ofpolarized short radiation pulses can be applied to a surface of thesubstrate while the surface is in contact with a fluid, e.g., a liquidsuch as water. Subsequently, the irradiated surface is exposed to aplurality of polarized short radiation pulses having a differentpolarization while the surface is contact with a fluid, e.g., a liquid.In many embodiments, this two-step irradiation process can causegeneration of nanometer-sized features in a top surface layer of thesubstrate, e.g., by breaking up the features formed during the firstradiation step into smaller-sized features. The term “short radiationpulses,” as used herein, refers to pulses of electromagnetic radiationhaving a temporal duration in a range of about 10 femtoseconds (fs) toabout a few hundred (e.g., 500 ns) nanoseconds (ns), preferably in arange of about 20 fs to about 500 fs (e.g., 100 fs).

With reference to a flow chart 10 of FIG. 1, in one exemplary embodimentof a method according to the teachings of the invention for processing asubstrate, e.g., a semiconductor substrate, in a step 12, at least aportion of the substrate surface is exposed to a fluid, for example, bydisposing a layer of the fluid over that portion. In many cases, thefluid comprises a liquid, such as water, alcohol or any other suitablefluid.

In another step 14, the substrate portion that is in contact with thefluid is exposed to one or more short polarized radiation pulses (e.g.,laser pulses) so as to modify its surface topography. In manyembodiments, the radiation pulses exhibit a linear polarization along aselected direction, though in other embodiments radiation pulses havingcircular polarization can be employed.

The radiation pulses can have temporal durations (pulse widths) in arange of about 20 fs to about a few hundred ns, and more preferably in arange of about 50 fs to about 500 fs. In this exemplary embodiment, thecenter wavelength of the pulses is chosen to be about 800 nm. Moregenerally, radiation wavelengths in a range of about 200 nm to about 800nm can be employed. The pulse energies can be in a range of about 10microjoules to about 100 millijoules.

In many embodiments, the fluid is selected to be substantiallytransparent to radiation having at least one wavelength component in arange of about 200 nm to about 800 nm. Further, the thickness of thefluid layer is preferably chosen so as to ensure that it would notinterfere with the radiation pulses (e.g., via excessive self-focusingof the pulses) in a manner that would inhibit irradiation of thesubstrate surface. While in this embodiment water is selected as thefluid, in other embodiments other suitable polar or non-polar liquidscan be employed. Some examples of such liquids include, withoutlimitation, alcohol and silicon oil. In other embodiments, the fluid cancomprise a gas, e.g., SF₆, N₂ or air.

In many embodiments, the radiation pulses are focused onto thesubstrate's surface, e.g., in a direction normal to the surface, suchthat the radiation fluence on the surface is greater than about 2 kJ/m².In general, in many embodiments, the radiation fluence can be less thanabout 40 kJ/m², e.g., in a range of about 2 kJ/m² to about 30 kJ/m².Further, in many embodiments, the substrate can be moved relative to theradiation and/or the radiation beam can be moved relative to thesubstrate's surface so that different portions of the substrate surfacecan be exposed to the radiation pulses.

In this embodiment, the substrate is translated relative to theradiation beam while irradiating the substrate surface to short laserpulses such that a portion of the surface is exposed to the laserpulses—in other cases a radiation beam can be moved over the substratewhile the substrate remains stationary. Generally, the repetition rateof the laser pulses and the translational speed of the substrate areselected such that each area of the surface is exposed to a number ofpulses ranging from 1 pulse to about 2500 pulses, and more typically afew hundred pulses.

In some embodiments, the teachings of U.S. Published Patent ApplicationNo. 2006/0079062 entitled “Femtosecond Laser-Induced Formation ofSubmicrometer Spikes On A Semiconductor Substrate,” which is hereinincorporated by reference in its entirety, can be used to carry outirradiation of the substrate in the aforementioned step 14.

The radiation pulses can modify the surface topography to generate aplurality of surface features, e.g., microns-sized and/orsubmicron-sized features on the surface. The type of surfacemodifications generated by the exposure of the surface to the radiationpulses in step 14 can depend, at least in part, on the wavelength of theradiation pulses as well as their fluence. By way of example, theexperimental results discussed further below show that in some cases theexposure of a silicon substrate to 800 nm, 100 femtosecond pulses (about200 laser pulses per area) having a linear polarization can result indifferent substrate surface modifications based on the fluence of thepulses.

For example, in a high fluence regime (e.g., a fluence greater thanabout 10 kJ/m²) micrometer-scale structures and submicrometer-sizedholes can be formed on a silicon substrate, as shown in FIG. 2A. In amedium fluence regime (e.g., a fluence in a range of about 4 kJ/m² toabout 10 kJ/m²), a plurality of straight ripples can be formed on asilicon substrate, as shown in FIG. 2B, with the ripple spacing beingsubstantially equal to the wavelength of incident pulses. The long axisof the ripples shown in FIG. 2B is substantially perpendicular to thedirection of the polarization of the laser pulses. In a low fluenceregime (e.g., a fluence in a range of about 2 to about 4 kJ/m²),straight ripples can be formed on a silicon substrate, which in thiscase show a spacing of about 120 nm, as shown in FIG. 2C. FIGS. 2A-2Care provided only for illustrative purposes, and it should be understoodthat in other cases the shapes and the sizes of the features formed on asubstrate by exposing it to radiation pulses in step 14 of an embodimentof the method of the invention can be different than those depicted inthose figures.

Referring again to the flow chart of FIG. 1, in another step 16, thesubstrate surface that was structured via exposure to the radiationpulses in step 14 is irradiated again, while the substrate surface is incontact with a fluid, e.g., the same fluid utilized in step 14 or adifferent fluid, with a plurality of short laser pulses having apolarization that is different than that of the radiation pulsesutilized in step 14. By way of example, the radiation pulses applied tothe substrate in the subsequent step 16 can have a linear polarizationthat is rotated by 90 degrees relative to a linear polarization employedin step 14. In other cases in which the radiation pulses employed instep 14 exhibit a circular polarization, the radiation pulses utilizedin step 16 can have a circular polarization with an opposite sense(e.g., when the circular polarization is step 14 is clockwise, thepolarization of the pulses in step 16 can be counter-clockwise).

Similar to step 14, the central wavelength of the laser pulses employedin step 16 can be generally in a range of about 400 nm to about 800 nm.In this embodiment, the wavelength is assumed to be 800 nm. Further, inmany embodiments, the radiation pulse widths can be in a range of about10 fs to about a few nanoseconds, and more preferably in a range ofabout 50 fs to about 500 fs. Although in general the fluence of theradiation pulses utilized in step 16 at the substrate surface can be ina range of about 2 kJ/m² to about 30 kJ/m², in many cases the fluence ofthe radiation pulses utilized in step 16 is less than the respectivepulses utilized in step 14, e.g., by a factor in a range of about 10% toabout 50%.

While in this embodiment, the fluid that is in contact with thesubstrate surface in step 16 is the same type of fluid as that employedin step 14, in other embodiments, two different types of fluids can beemployed in steps 14 and 16.

The second irradiation step 16 of the substrate surface with radiationpulses having a different polarization can cause the break-up of surfacefeatures generated by the first irradiation step 14 into finer-sizedfeatures. By way of example and as discussed further below in theexperimental section, the irradiation of the structured siliconsubstrate surface shown in FIG. 2C, while the surface is in contact withwater, by another set of 100 fs, 800 nm laser pulses (200 pulses perarea) at a fluence of 0.5 kJ/m² with a linear polarization that isrotated by 90 degrees relative to the polarization used to initiallygenerate the surface ripples can cause the surface ripples to break upinto nanometer-sized structures, as shown in FIG. 3A-3D. Morespecifically, FIGS. 3A and 3B are SEM images of the surface obtained ina direction normal to the surface at different resolutions while FIG. 3Cis another SEM image of the surface obtained from a different vantagepoint and FIG. 3D is a cross-sectional SEM image of the structuredsubstrate. In this case, the nanometer-sized structures are in the formof nanometer-sized rods, which are about 50 nm in diameter and up toabout 400 nm tall. In other cases, the shapes and the sizes of thestructures can be different.

In this manner, the two-step irradiation of the substrate surface withshort laser pulses having different polarizations (orthogonalpolarizations in the above embodiment) can result in the formation offine (in many cases nanosized) surface features. By way of example, insome cases the nano-sized features can include columnar spikes (e.g.,rods) extending from a base to a tip that protrude above the substratesurface. In some cases, the nano-sized spikes can have heights less thanabout 1 micron, or less than about 500 nm with a diameter in a range ofabout 50 nm to about 500 nm.

In some embodiments, the surface density of the nanosized structures,i.e., the number of nanosized structures per unit surface area, formedin a substrate surface layer via the aforementioned two-step irradiationprocess can be greater than about 10⁹ cm⁻², e.g., in a range of about5×10⁹ cm⁻² to about 5×10¹⁰ cm⁻².

In some embodiments, the nanosized structures are formed in a topsurface layer having a thickness, e.g., in a range of about 1 micrometerto about 100 micrometers.

Without being limited to any particular theory and as discussed furtherbelow, the formation of the nanosized structures in many embodiments caninvolve several processes: refraction of laser light in highly excitedsilicon, interference of scattered and refracted light, rapid coolingdue to contact with a fluid, and capillary instabilities.

As noted above, a variety of fluids can be utilized in the steps 14 and16 in the above method for structuring a substrate surface. As notedabove, in some cases the fluid can be a liquid, such as water, alcohol,or an aqueous solution, e.g., one having an electron-donatingconstituent. For example, a solution of sulfuric acid can be applied toat least a portion of the substrate followed by irradiating that portionwith short pulses (e.g., femtosecond pulses) to cause a change not onlyin surface topography in a manner described above but also generatesulfur inclusions within a surface layer of the substrate. In somecases, the fluid can be a gas, e.g., sulfur hexafluoride, or air. Moregenerally, the fluid can include a dopant compound, at least a portionof which can be incorporated into a surface layer of a substrate whosesurface is irradiated with the short radiation pulses.

By way of example, FIG. 4 schematically depicts an apparatus 18 that canbe utilized to practice a method according to the invention forstructuring a substrate, such as the aforementioned embodiment discussedabove with reference to the flow chart 10. The apparatus 18 includes aTitanium-Sapphire (Ti:Sapphire) laser 20 that generates laser pulseswith a pulse width of 80 femtoseconds at 800 nm wavelength having anaverage power of 300 mW and at a repetition rate of 95 MHz. The pulsesgenerated by the Ti:Sapphire laser 20 are applied to a chirped-pulseregenerative amplifier 22 that, in turn, produces 0.4 millijoule (mJ),100 femtosecond pulses at a wavelength of 800 nm and at a repetitionrate of 1 kilohertz.

The femtosecond laser pulses are directed via a mirror 24 to a lens 26that in turn focuses the laser pulses onto a surface of a sample 28(e.g., a silicon wafer) disposed on a 3-dimensional translation system30 within a vacuum chamber 32. A glass liquid cell 34 is coupled to thestage over the sample so as to allow a sample surface to have contactwith the fluid (e.g., water) contained within the cell. Thethree-dimensional stage allows moving the sample relative to the laserpulses for exposing different portions of the surface to radiation. Thevacuum chamber can be utilized to pump out air bubbles in the fluid.Alternatively, the processing of the sample can be performed withoututilizing a vacuum chamber. The aforementioned U.S. Published PatentApplication No. 2006/0079062 entitled “Femtosecond Laser-InducedFormation of Submicrometer Spikes On A Semiconductor Substrate,”discloses a variation of the apparatus 18 that is suitable forirradiating a substrate with laser pulses at a wavelength of 400 nm.

In some embodiments, a substrate surface is initially roughened (e.g.,by generating a plurality of micro-sized structures within a surfacelayer) before applying short radiation pulses in the above two-stepprocess to form nano-sized features on the roughened surface. By way ofexample, a substrate surface can be irradiated in presence of abackground gas by short laser pulses to form micron-sized structureswithin a top surface layer of the substrate (e.g., a top layer having athickness in a range of about 1 micrometer to about 100 micrometers).Subsequently, such a microstructured surface can be subjected to theaforementioned two-step irradiation process while the surface is incontact with a liquid, such as water, to generate nanometer-sizedfeatures in the microstructure surface layer, e.g., superimposed on themicrostructured features.

By way of example, in some embodiments, a substrate (e.g., asemiconductor) surface can be irradiated with laser radiation pulseshaving pulse widths in a range of about 50 femtoseconds to about a fewhundred nanoseconds (e.g., in a range of about 100 femtoseconds to about500 femtoseconds) and having a central wavelength in a range of about400 nm to about 800 nm while the surface is in contact with a gas, e.g.,nitrogen, or sulfur hexafluoride, to generate a plurality of micro-sizedspikes in a top surface layer. Further details regarding the formationof such micron-sized features can be found, e.g., in U.S. PublishedApplication No. 2003/0029495 entitled “Systems and Methods For LightAbsorption And Field Emission Using Microstructured Silicon,” and U.S.Pat. No. 7,057,256 entitled “Silicon-Based Visible and Near-InfraredOptoelectric Devices,” both of which are herein incorporated byreference in their entirety.

Subsequently, the microstructured surface can be placed in contact witha liquid, such as water, and exposed in a two-step process, such as thatdiscussed above, to two sets of short laser pulses, where thepolarization of the pulses in one set is different than the polarizationof the pulses in the other set (e.g., the two sets can comprise linearlypolarized pulses, where the direction of polarization in one set isorthogonal to the direction of polarization in the other set). Forexample, the short radiation pulses can have pulse widths in a range ofabout 50 fs to about 500 fs at a wavelength in a range of about 400 nmto about 800 nm. Similar to the previous embodiments, the substratesurface can be moved relative to the laser beam and/or the laser beamcan be moved relative to the substrate surface to expose each area ofthe substrate surface to one or more pulses (e.g., a few hundredpulses). In this manner, combination of micron-sized and nano-sizedfeatures can be generated in a surface layer of a substrate.

In some embodiments, rather than utilizing radiation at two differentpolarizations in a two-step process such as those discussed above,circularly polarized radiation is employed in a one-step process (aprocess that utilizes only one polarization) to generate micron-sized,and preferably submicron-sized, structures with a top surface layer of asubstrate, e.g., a semiconductor, a metal or an insulator). Morespecifically, in one such embodiment, a substrate surface can be exposedto a fluid, e.g., a gas or a liquid such as water, alcohol or any othersuitable liquid, while the surface is illuminated with short radiationpulses having circular polarization of one sense (e.g., clockwise orcounter-clockwise). The short radiation pulses can have pulse widths,e.g., in a range of about 10 femtoseconds to about a few hundrednanoseconds (e.g., 200 ns) and more preferably in a range of about 100fs to about 500 fs.

In some cases, the substrate is translated relative to the radiationpulses, or vice versa, so as to apply a plurality of pulses to eachirradiated surface location (e.g., a number of pulses in a range ofabout 10 to about a few thousands (e.g., 2500), and more typically a fewhundred (e.g., 200) pulses). The radiation pulse can have a fluence,e.g., 2 kJ/m² to about 40 kJ/m², and an energy in a range of about,e.g., about 10 microjoules to about 100 millijoules. Further, in someembodiments, the central wavelength of the pulses can be, e.g., in rangeof about 200 nm to about 800 nm, though other wavelengths can also beutilized.

The processing of a substrate by such circularly polarized pulses canlead to formation of micron-sized and preferably sub-micron sizedstructures in a top surface layer of the substrate, e.g., a surfacelayer having a thickness in a range of about 1 micrometer to about 100micrometers.

The methods of the invention, such as the embodiments discussed above,can be applied to substrates formed of different materials. By way ofexample, they can be applied to semiconductor substrates, such assilicon, germanium, CdTe, CdSe, and GaAs.

By way of further illustration, the following examples provideexperimental results obtained by irradiating a silicon substrate incontact with water in a two-step process in accordance with theteachings of the invention. It should be understood that the followingexamples are provided only for illustrative purposes and are notintended to necessarily indicate optimal results that can be obtained bypracticing the methods of the invention.

Example 1

In each of a number of experiments, a single crystalline siliconsubstrate sample was placed in a glass container filled with water,which was mounted on a three-axis translation stage. The sample wasirradiated by a train of 100-fs, 800 nm laser pulses at a repetitionrate of 1-kHz from an amplified Ti:Sapphire laser with laser energies upto about 400 microJoules (μJ). The laser pulses were focused by a 0.25-mfocal length lens to travel through approximately 10 mm of water beforestriking the substrate surface at normal incidence. The focal point wasapproximately 10 mm behind the substrate surface and the spatial profileof the laser spot was nearly Gaussian. The sample was translated in adirection perpendicular to the laser beam.

The aforementioned FIG. 2A-2C show scanning electron microscope (SEM)images of structures formed on three silicon substrate in water at threedifferent laser fluences using an average irradiation of 200 laserpulses per area. As shown in FIG. 2A, at a laser fluence of 25 kJ/m²,micrometer-sized structures and submicrometer-sized holes were formed ina substrate surface layer. As shown in FIG. 2B, at a lower fluence of 5kJ/m², which lies in a medium fluence range of about 4 to about 10kJ/m², a plurality of ripple-like structures were formed with spacingbetween adjacent ripples substantially equal to the laser wavelength (inthis case 800 nm). The ripples were substantially straight with theirlong axis perpendicular to the laser polarization. As shown in FIG. 2C,at a lower fluence of about 3 kJ/m², which lies in a low-fluence rangeof about 2 to about 4 kJ/m², straight ripples are formed with a spacingof about 120 nm with the long axis of the ripples perpendicular to thelaser polarization.

The surface ripples shown in FIG. 2C were irradiated again with 100 fs,800 nm laser pulses (an average of 200 pulses per area) while thesubstrate surface remained in contact with water. The polarization ofthe laser pulses was, however, rotated by 90° relative to thepolarization originally used to form the ripples. In other words, thepolarization of the laser pulses in the second irradiation step wassubstantially parallel with the long axis of the ripples. The secondirradiation step caused the break-up of the ripples into thenanometer-scale structures shown in FIG. 3A-3D. These structures includenanometer-sized rods, which are about 50 nm in diameter and up to about400 nm tall, which substantially uniformly cover the surface.

Without being limited to any particular theory, the three distinctsurface morphologies presented in FIGS. 2A-2C after the firstirradiation step can be attributed to different types of laserinteraction with the silicon/water system: ultrafast melting andresoldification at low fluence, ultrafast melting and ablation at mediumfluence, and ultrafast melting, ablation and bubble cavitation at highfluence.

For example, the straight ripples formed approximately normal to thelight polarization in the medium fluence region with a spacingsubstantially equal to the laser wavelength can be attributed to theinterference between light that is scattered at a roughened surface andlight that is refracted at the surface: the light interferes just belowthe surface and is absorbed in a non-uniform periodic ripple pattern.For an optically thick molten layer, the periodic absorption patternexcites surface waves in the molten layer and the ripples are “frozen”into the surface upon solidification.

In the low fluence regime, the front portion of a laser pulse can excitea large number of electrons, thus causing an increase in the index ofrefraction in a thin surface layer of silicon. Within this high-indexlayer, the effective wavelength of the refracted and scattered light canbe reduced, thus causing a reduction in the periodicity of theinterference pattern. Approximately one picosecond after the laser pulsestrikes the surface, ultrafast melting produces a liquid layer whoseevolution is dictated by surface-wave driven growth dynamics. Theperiodic absorption excites nanometer-scale surface waves in the liquidlayer, which upon resolidification are “frozen” into nanometer-scaleripples.

In the high fluence regime, the irradiation fluence can be above theablation threshold, and consequently a thin layer of high-index siliconis ablated away and no nanometer-scale ripples are observed.

Generally, the surface morphology after laser pulse irradiation in manycases can be due to an interplay between surface wave dynamics and thelifetime of a molten layer generated by the pulses. Taking into accountthat the melt depth can be large compared to 120 nm and includingdamping due to viscosity, a lifetime of approximately 1 ns can beobtained for a surface wave with a wavelength of 120 nm. For surfacesirradiated in a gas, the lifetime of the melt can be longer than about30 ns, that is, much longer than the lifetime of a surface wave with awavelength of 120 nm. As such, by the time the surface resolidifies, anynanometer-scale surface waves have died out and been replaced by surfacewaves with longer wavelengths and longer lifetimes. The thermalconductivity of water is, however, over one order of magnitude higherthan that of a typical gas, and water vaporizes and dissociates on thesubstrate surface. The larger thermal conductivity of water and itsvaporization and dissociation can result in a large heat transfer out ofthe molten layer, and so the lifetime of the molten layer in water canbe less than about 1 ns. The presence of water in contact with thesubstrate surface and the associated increase in the cooling rate of themolten layer can allow for the nanometer-scale ripple patterns to befrozen into the surface before they die away.

As noted above, FIGS. 3A-3D show that during the second irradiation stepof the nanometer-scale ripples shown in FIG. 2C, nanometer-scale rodsform along the ripples. Although the second irradiation is at a muchlower fluence than the first irradiation step, the laser pulses causemelting of the nanometer-scale ripples as the radiation absorptance of arippled surface can be higher than that of a smooth surface. Once theyare molten, the ripples tend to break up into beads, presumably becausea molten half-cylinder can be unstable. Similar to the formation of theripples, the size of the beads can be set by the interference betweenincident and scattered laser light below the surface, which can producea periodic absorption pattern along the long axis of the ripples. Oncethe ripples break up into beads, subsequent laser irradiation cansharpen the beads into rods through preferential removal of materialaround the beads by laser-assisted ablation.

Example 2

In another set of experiments, a flat silicon substrate surface wasinitially irradiated by 100 fs, 500-microJoule, 800 nm laser pulseswhile the surface was in contact with SF₆ gas in a manner disclosed inan article entitled “Microstructuring of silicon with femtosecond laserpulses” by Her et al., published in Appl. Phys. Lett. 73, 1673 (1998),which is herein incorporated by reference, to form micron-sized spikesin top surface layer. The microstructured surface was subjected to thetwo-step irradiation process discussed above, where the polarization inone step was orthogonal to that in the other step. These SEM images ofthe resulting surface at different resolutions, presented in FIGS. 5A,5B, and 5C, show nanometer-sized rods that cover the entiremicrostructured surface, protruding substantially normal to the surfaceregardless of the original surface morphology.

FIGS. 5A-5C provide further evidence of the role of a high-index, moltensilicon layer in the formation of nanometer-sized ripples and rods: thenanometer-sized rods cover the entire surface of each micrometer-sizedspike and are normal to the surface of the spike. The high-index layerat the surface of each micrometer-sized spike strongly refracts thelaser light the refracted laser light becomes nearly perpendicular toevery part of the solid silicon surface, which creates a temperaturegradient approximately perpendicular to the surface. Consequently, thenano-sized rods are formed approximately normal to the sample surface.

Those having ordinary skill in the art will appreciate that variouschanges can be made to the above embodiments without departing from thescope of the invention.

1-32. (canceled)
 33. A semiconductor substrate, comprising: a pluralityof nanosized structures disposed in a surface of the substrate, whereinsaid nanosized structures exhibit a surface density in a range of about5×10⁹ cm⁻² to about 5×10¹⁰ cm⁻².
 34. The semiconductor substrate ofclaim 33, wherein said substrate has a thickness in a range of about 1micrometer to about 100 micrometers.
 35. The semiconductor of claim 33,wherein said substrate comprises a surface layer having a plurality ofdopant inclusions, wherein said surface layer extends from saidnanosized structures to a depth of the substrate.
 36. The semiconductorsubstrate of claim 33, wherein said nanosized structures have a heightless than about 500 nm.
 37. The semiconductor substrate of claim 33,wherein said nanosized structures have a height in a range of about 50nm to about 500 nm.
 38. The semiconductor substrate of claim 33, whereinsaid nanosized structures comprise a plurality of nanosized rods. 39.The semiconductor substrate of claim 33, wherein said substratecomprises any of silicon, germanium, CdTe, CdSe or GaAs.
 40. Thesemiconductor substrate of claim 33, wherein said nanosized structuresare formed by exposing the substrate to laser radiation.
 41. Thesemiconductor substrate of claim 33, wherein said laser radiationcomprises a plurality of short radiation pulses.
 42. The semiconductorsubstrate of claim 41, wherein said short radiation pulses have atemporal duration in a range of about 10 fs to about a few hundrednanoseconds.
 43. The semiconductor substrate of claim 42, wherein saidshort radiation pulses have a temporal duration in a range of about 100fs to about 1 ps.
 44. The semiconductor substrate of claim 43, whereinsaid short radiation pulses have a temporal duration in a range of about100 fs to about 500 fs.